Wire bonding is generally a means of electrical connection between a semiconductor chip and a substrate. The substrate may, for example, be a printed circuit board (PCB) or a lead frame. Wire bonding typically involves using gold (Au) wire, aluminum (Al) wire, copper (Cu) wire, silver (Ag) wire, or a combination of alloy wire to form the electrical connection. One type of wire bond process, for example, is the Stand Off stitch on ball Bonding (SSB). An advantage of SSB is that it provides better loop control and can achieve a lower wire loop profile.
Au wire is commonly used as a form of electrical connection between the semiconductor chip and the substrate. Typically, the Au wire is bonded to an Al bond pad formed on the chip at one end, and bonded to the substrate at the other end. During bonding, the Au and Al inter-diffuse into each other and may result in high electrical resistance and high heat generation. This may then lead to low bonding reliability and device performance. Also, the poor heat dissipation characteristic of gold materials may cause overheating in the IC assembly.
Furthermore, Au materials have low tensile strength and may result in poor wire sagging, poor wire sweeping performance, poor wire loop profile and instability for long wires, during packing encapsulation. Also, in Au wire bonding, a process of Ni and Au coating on the substrate is required in order to achieve an acceptable electrical connection between the Au wire and the substrate.
In view of the foregoing disadvantages and the expensive cost of gold, other materials, for example, copper (Cu), have been contemplated for wire bonds. Particularly, copper, which has better conductivity as compared to gold materials, thereby increasing device power rating and improving package heat dissipation.
A challenge of using copper in wire bonding is that the bond pad surface on the chip or the lead finger surface on the substrate may have oxidized material coated thereon, which may decrease bonding reliability. For example, when wire bonding to a Cu bond pad, the Cu bond pad oxidizes readily to form a layer of oxide on the bond pad surface. The oxide layer prevents effective bonding between the wire and the Cu bond pad.
From the foregoing discussion, it is desirable to improve the performance of wire bond, such as SSB process for copper wire bonding and copper wire bonding through OSP surface.